This present invention relates to manufacture of electrochemical cells. More particularly, the present invention provides a method and resulting device for forming elements of a thin film solid-state electrochemical cell. Merely by way of example, the invention has been provided with use of lithium based cells, but it would be recognized that other materials such as zinc, silver, copper and nickel could be designed in the same or like fashion. Additionally, such batteries can be used for a variety of applications such as portable electronics (cell phones, personal digital assistants, music players, video cameras, and the like), power tools, power supplies for military use (communications, lighting, imaging and the like), power supplies for aerospace applications (power for satellites), and power supplies for vehicle applications (hybrid electric vehicles, plug-in hybrid electric vehicles, and fully electric vehicles). The design of such batteries is also applicable to cases in which the battery is not the only power supply in the system, and additional power is provided by a fuel cell, other battery, IC engine or other combustion device, capacitor, solar cell, etc.
The intercalation of a variety of electron donors, including Li-ions, into amorphous or crystalline materials, such as transition metal compounds and graphite has been widely studied and used as a principal mechanism of rechargeable batteries. When a Li-ion cell is charged, the positive material is oxidized and the negative material is reduced. In this process, Li-ions are de-intercalated from the positive material and intercalated into the negative material. The reverse happens on discharge. Of particular interest to the field of Li-ion batteries recently is the work on developing intercalation compound as cathode materials. However, these amorphous or crystalline intercalation compounds often have insufficient ionic/electronic conductivity to be used in battery electrodes alone.
Conventional method of improving electrode conductivity is mixing or coating the active materials (i.e. intercalation compounds) with conductive additives, such as a microbead mesophase carbon, artificial graphite or milled graphite fiber. Most Li-ion batteries are produced this way today. The use of composite electrodes has been one of the key problems to Li-ion battery researchers as it is linked to battery failure mechanisms. The inhomogeneous mixture in the electrode caused non-uniform charge distribution and localized stress and heat generation, often leads to the limited cycle life of batteries. Furthermore, addition of binder and conductive materials reduce energy density of the battery.
An alternative method of enhancing conductivity in amorphous and crystalline intercalation compound is changing the material's own composition by doping or substituting with other elements. Changing the doping is to improve the functional electrical properties of electron transport and electric or ion to improve the structural stability of materials. This doping method can be readily done with physical vapor deposition (PVD) for solid-state battery manufacturing using additional ion source to the deposition processes.
Solid-state electrolytes for Li-ion batteries including Li-ion conducting glassy materials, best represented by LiPON (Lithium Phosphorus Oxynitride), have been fabricated by physical vapor deposition techniques. LiPON films are typically deposited by sputtering technique using lithium phosphate target in nitrogen plasma, resulting enhanced ionic mobility and good ionic conductivity at 10−6-10−7 S/cm due to the formation of additional phosphate cross-linking by nitrogen ions. However, the deposition rate from sputtered LiPON is limited to only 1-2 Å/s. Other techniques using higher energy sources, such as electron beam evaporation attempt to achieve higher deposition rate, but their yields to obtain defect-free thin films are low. The key challenge to the high volume production of thin film batteries and the solid-state electrolyte is achieving a high deposition rate and preventing the defect formation caused by the higher energy transfer during the processing.
Prior art with conventional battery materials manufacturing has been mostly limited to the particulate materials, and its synthesis method by chemical routes involving high temperature processes, which is not suitable for roll-to-roll solid-state battery manufacturing. Prior art with solid-state thin film materials has been limited to a small number of deposition techniques and enhancement of cathode and electrolyte conductivity has been challenging issue, as well as faster process rate and higher production yield. For roll-to-roll processes using polymer substrate, lower temperature PVD process is desired for enhancing materials ionic/electronic conductivity. Furthermore, PVD process that we describe in this patent has advantages over the chemical doping methods, in terms of tunability, processibility, and cost.
Accordingly, it is seen that there exists a need for a method and materials to produce an improved package of a large scale, high capacity solid-state battery.